Definition · Plain-language
Energy
Energy is the capacity of a system to do work or cause change, existing in many interchangeable forms.
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The capacity to do work
In physics, energy is defined as the capacity to do work — to apply a force over a distance — or, more broadly, to cause change. Anything that can heat, move, light or transform something possesses energy. It is a scalar quantity, measured in joules in the SI system, where one joule is the energy used when a force of one newton acts over one metre. Energy is one of the most fundamental concepts in science precisely because almost every physical process can be described as energy being transferred or converted.
The main forms of energy
Energy appears in many forms. Kinetic energy is the energy of motion; potential energy is stored energy due to position or state, such as a raised weight or a stretched spring. Thermal energy is the energy of the random motion of particles, felt as heat. Chemical energy is stored in the bonds of fuels and food, electrical energy in moving charge, nuclear energy in the nucleus of atoms, and radiant energy in light and other electromagnetic waves. These forms are constantly converting into one another all around us.
Conservation and transformation
The cornerstone principle is the conservation of energy: in an isolated system, energy can change form but the total amount never changes. Burning petrol turns chemical energy into kinetic energy and heat; a falling object turns potential energy into kinetic energy; a solar panel turns radiant energy into electrical energy. Even when energy seems to disappear, as friction slows a moving object, it has simply become heat. Einstein extended the principle by showing mass itself is a form of energy, linked by E = mc².
Key facts
At a glance
- Definition: the capacity to do work or cause change
- SI unit: the joule (J)
- Quantity type: a scalar — magnitude only
- Main forms: kinetic, potential, thermal, chemical, electrical, nuclear, radiant
- Conservation: total energy in an isolated system stays constant
- Mass–energy: mass is a form of energy, linked by E = mc²
Common misconceptions
What people often get wrong
Often heard: Energy gets used up and disappears.
Actually: Energy is conserved — it is never destroyed, only converted to another form. What we call "using" energy is really transforming it, often into low-grade heat that spreads out and becomes less useful.
Often heard: Energy and power mean the same thing.
Actually: Energy is the total capacity to do work, measured in joules; power is the rate at which energy is transferred or used, measured in watts. A device can use a lot of energy slowly or a little quickly.
Often heard: Heat and temperature are the same kind of energy.
Actually: Temperature measures how hot something is, the average energy of its particles, while heat is the thermal energy transferred between objects. A small very hot object can hold less heat energy than a large warm one.
